Ultrafast photoresponse of metallic and semiconducting single-wall carbon nanotubes

Utilizing a transient absorption (TA) technique based on a chirp-corrected broadband probe, we have studied the ultrafast photoresponse of dispersed HiPco single-wall carbon nanotubes (SWNTs) over the range of 440--1050 nm for excitation in the range of 430--1700 nm. While both metallic and semiconducting SWNTs show transient bleaching at their ${M}_{11}$ and ${S}_{11}$ energies for excitation above these energies, metallic SWNTs uniquely exhibit a photoresponse to sub-${M}_{11}$ excitation. We observe a TA spectral response for metallic SWNTs which is consistent with thermal broadening of the ${M}_{11}$ transition bands. In contrast to metallic SWNTs, specific semiconducting SWNTs exhibit transparency for sub-${S}_{11}$ excitation, in accord with the expected zero density of states below the first interband transition. We report the observation of a long-lived $(\ensuremath{\tau}>1\phantom{\rule{0.3em}{0ex}}\mathrm{ns})$ transient bleach component for three semiconducting SWNT species.